1. Field of the Invention
This invention relates to a semiconductor laser apparatus producing laser light with a stabilized oscillation wavelength, which can be used as a coherent light source in an optical communication system, an optical measuring system, an optical information processing system, etc.
2. Description of the Prior Art
With the enlarged use of semiconductor lasers in fields such as optical communication, optical measurement, optical information processing, etc., semiconductor lasers producing laser light with a stabilized oscillation wavelength are required as a light source. Semiconductor lasers ordinarily produce laser light having an oscillation wavelength that continuously or discontinuously varies with a variation in temperatures and/or current. This variation in an oscillation wavelength causes optical output noise, which causes a decrease in the performance of the optical system containing these semiconductor lasers therein. In order to eliminate these problems, external resonator type semiconductor lasers have been developed. FIG. 5 shows a conventional external resonator type semiconductor laser in which a semiconductor laser device 1 is mounted on a base 2 in such a manner that the epitaxial growth layer side of the laser device 1 faces the base 2 so as to improve the radiation of heat. Laser light emitted from the front facet 13 of the active layer 10 of the laser device 1, which is located at a distance of several .mu.m or less from the base 2, is radiated to the outside through a window 5. The base 2 is fixed to a table 6 that constitutes a housing together with the window 5 and the side wall 7. A reflector 3 is also mounted on the base 2. A part of the laser light emitted from the light-emitting rear facet 12 of the laser device 1 is reflected by a reflecting face 41 of the reflector 3 and then returns to the laser device 1. Wiring, functioning as an electric path, is omitted from FIG. 5 in order to simplify the drawing. A semiconductor chip can be used as the reflector 3, the reflecting face 41 of which is formed by, for example, coating a cleaved facet of the chip with a multilayered reflecting film made of a metal such as Au or a dielectric substance.
In the semiconductor laser device 1 with the above-mentioned structure, an external longitudinal mode .lambda..sub.e (=2d (m.sub.e +1/2)), which depends upon the distance d between the light-emitting rear facet 12 and the reflecting face 41, arises. Accordingly, the semiconductor laser device 1 stably oscillates in the longitudinal mode near the peak of a gain distribution in which the internal longitudinal mode .lambda.(=2nl/m) depending upon the internal cavity length l agrees or nearly agrees with the above-mentioned external longitudinal mode .lambda..sub.e, wherein m and m.sub.e are the integer and n is the effective refraction of the semiconductor laser waveguide. According to the experimental results obtained by the inventors of this invention, when the external cavity length d is 50 .mu.m, a semiconductor laser apparatus oscillating laser light in a stable longitudinal mode in a wide temperature range over a 31.degree. C. span at a fixed optical output power as shown in FIG. 6 is obtainable. The characteristics of such a semiconductor laser apparatus meet the requirements necessary for the optical system. In general, in order to stabilize the intensity of laser light emitted from semiconductor lasers, the semiconductor lasers must be provided with a monitoring structure in which the intensity of laser light from the light-emitting front facet is monitored by detecting the laser light from the light-emitting rear face by the use of a photodetector. However, the semiconductor laser with the structure shown in FIG. 5 cannot be provided with such a monitoring function because the laser light from the light-emitting rear facet 12 is intercepted by the reflector 3 before arriving at a photodetector. Thus, the semiconductor laser must have an optical apparatus leading a part of the laser light from the light-emitting front facet to the photodetector. FIG. 7 shows such an optical apparatus 8, which comprises a collimating lens 81 for collimating the laser light beams 131 from the light-emitting front facet 13, a beam splitter 82 for splitting the collimated laser beams into two components 132 and 133, a condensing lens 83 for condensing the light beam 132, and a condensing lens 84 for condensing the light beam 133. The light beam 132 condensed by the condensing lens 83 is used as a light source for reading and/or writing information, whereas the light beam 133 condensed by the condensing lens 84 is incident on an optical detector 85 for detecting the optical output power of the semiconductor laser. It is extremely difficult to dispose each of the optical means with high precision. Moreover, since a part of the laser light from the front facet of the semiconductor laser is used as a light for monitoring the intensity of laser light emitted from the front facet of the semiconductor laser, the optical output intensity of the semiconductor laser is weakened.